This invention relates to frequency synthesizers and is specifically concerned with synthesizers which produce only a very low level of unwanted noise in addition to a required output signal at a specified frequency. The output frequency of a frequency synthesizer is never completely pure, but possesses frequency modulation noise side bands which stem from the way the output frequency is synthesized and from the nature of the standard frequency source which is used by the synthesizer as a frequency reference. A standard frequency source commonly takes the form of a very stable crystal oscillator, and although such an oscillator generates a certain amount of noise, it is at a low level and is often acceptable. It is impracticable to produce a crystal oscillator which is capable of supporting a fundamental mode of oscillation at frequencies greatly in excess of a few tens of megahertz because the physical dimensions of the crystal cannot be reduced below a level at which it can be readily and reliably manufactured.
Thus the need for a frequency synthesizer which is able to operate at very much higher frequencies, e.g. of the order of several hundred megahertz, has been satisfied by multiplying the output frequency of the crystal oscillator by an appropriate factor. This process inevitably worsens the noise to a very great extent. For example, while a 10 MHz crystal oscillator can exhibit a very acceptable single side band noise figure of -170 dB/hertz at a given off-set frequency, the effect of multiplying the frequency by a factor of 50 results inevitably in a noise figure of -134 dB even if no additional sources of noise are introduced.